This invention relates to a method of manufacturing a magnetic recording medium used as a recording medium for an information processing apparatus and the like, and a method of manufacturing a substrate thereof.
Recently, a magnetic disk has been used as an information recording medium. The magnetic disk is structured by forming a thin-film, such as, a magnetic layer, on a substrate. In this event, an aluminum substrate or a glass substrate has been generally used as the substrate.
However, the glass substrate, which can further narrow a space (namely, a flying height with respect to a magnetic head) between a magnetic head and a magnetic recording medium in comparison with the aluminum substrate, has been gradually replaced by the aluminum substrate in accordance with high recording density in the recent years.
Such a glass substrate is generally manufactured by chemically strengthening to enhance strength and endure for impact when the glass substrate is mounted for a magnetic disk drive. Further, a surface of the glass substrate is polished with high accuracy so as to lower the flying height of the magnetic head to the utmost. Thereby, high recording density has been realized.
In the meantime, a thin-film head has been recently replaced by a magneto-resistive head (namely, MR head) in the magnetic head to realize the high recording density other than the glass substrate.
High surface flatness of the magnetic disk is required to realize a low flying height necessary for the above-mentioned high recording density. In addition, when the MR head is used, the surface of the magnetic recording medium must have high flatness from the viewpoint of thermal asperity.
When the magnetic disk has projections on the surface of the magnetic disk, the MR head is affected by the projections to generate heat for the MR head, and resistance value of the head is fluctuated to cause to occur an error operation for electro-magnetic conversion by the heat. This phenomenon is defined as the above-mentioned thermal asperity.
Further, even when the surface of the magnetic disk has the high flatness, if the surface of the magnetic disk has the projections which cause the thermal asperity, head crush brings about by the projections, and a magnetic film constituting the magnetic disk is peeled in the cause of the head crush. Thus, the projections give an adverse affect for the magnetic disk.
Thus, demand has been gradually enhanced about the high surface flatness of the magnetic disk to realize the low flying height and prevent the head crush and the thermal asperity. The substrate surface having the high flatness is finally required to obtain the high surface flatness of the magnetic disk. However, the high recording density can be no longer realized only by polishing the substrate surface with the high accuracy.
More specifically, even when the substrate surface is polished with the high accuracy, the high flatness can not be realized in case that contaminants are attached on the substrate. Although the contaminants have been naturally and conventionally removed, the contaminants, which have been placed on the substrate and conventionally have been permitted, cause a problem in a recent level with respect to the high recording density.
In this case, excessively small iron powder and stainless steel piece, which can not remove by the use of a normal washing process, are exemplified as this kind of contaminant. For example, it has been confirmed that when a chemical strengthening process is performed on the condition that particles, such as, the iron powders are attached on the glass substrate or that the particles are attached on the glass substrate in the chemical strengthening processing liquid, irons are strongly attached on the glass substrate to form island portions (namely, the projections) through oxidation reaction occurred in the chemical strengthening process and heat applied in the process.
It has been found out that when the thin-film, such as, the magnetic film is laminated on the glass substrate, the island portions (projections) are formed on the surface of the magnetic disk to prevent the low flying height and to occur the head crush and the thermal asperity.
Therefore, investigation has been fully made about a cause in which such fine iron powders are attached to the glass substrate. As a result, it has been confirmed that the iron powders are contained in a chemical strengthening chamber for performing the chemical strengthening process, and in particular, a large number of iron powders are contained in a chemical strengthening salt itself.
More specifically, when the number of the iron powders has been investigated for each generation factor, the number of the iron powders contained in the chemical strengthening salt itself before the chemical strengthening salt (sodium nitrate or potassium nitrate) is prepared to make the chemical strengthening processing liquid is excessively high.
Further, it has been found out that the chemical strengthening salt itself contains the other particles which give an adverse affect for the information recording medium by attaching to the glass substrate for the information recording medium.
Meanwhile, disclosure has been made about a technique for removing the iron powders contained in atmosphere of the chemical strengthening chamber for performing the chemical strengthening process and preventing the iron powders from contaminating the chemical strengthening processing liquid in Japanese Unexamined Patent Publication No. H10-194785.
Another disclosure has been made about a technique for removing the iron powders contaminated from the atmosphere in the chemical strengthening chamber into the chemical strengthening processing liquid by filtering the chemical strengthening processing liquid by the use of a filter having superior corrosion resistance to high temperature, such as, a microsieve (namely, wire cloth in which holes are opened by etching) in Japanese Unexamined Patent Publication No. H10-194786.
In this event, the former method is effective for removing the iron powders contained in the atmosphere in the chemical strengthening chamber for performing the chemical strengthening process.
Although the latter method has a constant effect, the number of the iron powders contained the chemical strengthening salt itself before making the chemical strengthening processing liquid is excessively high as mentioned above, and as a result, the latter method is not sufficiently effective for removing the iron powders.
Further, the latter method is not enough to remove the other particles which attach to the glass substrate for the information recording medium in the chemical strengthening process and give the adverse affect for the information recording medium.
Moreover, the chemical strengthening process is carried out by replacing ions contained in the glass by ions contained in original liquid for ion exchange, or distribution with respect to index of refraction is adjusted in a glass substrate for an electron device (including a glass substrate for a photomask, a glass substrate for a phase shift mask, or a glass substrate for an information recording medium, and hereinafter, will be used as the same meaning) or a glass substrate for an optical device in addition to above-mentioned glass substrate for the information recording medium.
In such glass substrates, the original liquid for ion exchange contains Fe and Cr, and thereby, the efficiency of ion exchange is lowered or the island portions are formed. For example, the island portions shield a light beam, and as a result, a desired characteristic may not obtained.